Optical Absorption Properties of Dispersed Gold and Silver Alloy Nanoparticles†

Jess Wilcoxon*Nanoscale Physics Research Laboratory, UniVersity of Birmingham, Birmingham B15 2TT, United Kingdom

ReceiVed: August 4, 2008; ReVised Manuscript ReceiVed: October 23, 2008

The oldest topic in nanoscience is the size-dependent optical properties of gold and silver colloids ornanoparticles, first investigated scientifically by Michael Faraday in 1857. In the modern era, advances inboth synthesis and characterization have resulted in new insights into the size-dependent absorbance of Auand Ag nanoparticles with sizes below the classical limit for Mie theory. In this paper we discuss the synthe-sis and properties of core/shell and nanoalloy particles of Au and Ag, compare them to particles of pure goldand silver, and discuss how alloying affects nanoparticle chemical stability. We show that composition, size,and nanostructure (e.g., core/shell vs quasi-random nanoalloy) can all be employed to adjust the opticalabsorbance properties. The type of nanostructurescore/shell vs alloysis reflected in their optical absorbancefeatures.

I. Introduction

The investigation of the dependence of the optical propertiesof dispersed metal colloids with nanometer scale dimensionson size, composition, and shape has a long and colorful history.1

Alchemists in early Roman periods utilized the unique opticalproperties of such noble metals as gold to make valuable objectssuch as the Lycurgus cup. This ornate goblet with embeddedcolloidal particles of gold and silver can be viewed in the Britishmuseum. Like thin films of gold later investigated by MichaelFaraday,2 this antiquity has a green tint in reflected light but ared tint when viewed in transmission. The strikingly deep colorsfound in medieval stained glass are another example of the useof colloidal metals to provide inorganic pigments for color. Thedurability and lack of fading with time of such inorganicpigments compared to organic ones is a major advantage.

Michael Faraday made the first scientific investigation of thecolor of colloidal gold with the goal of discovering why verythin films of gold have different colors when viewed in reflectedor transmitted light and how these colors depend on thickness.2

His estimates of the nanometer scale thickness of such filmswere surprisingly good given the technique available at that time.His solutions of colloids also did not have the gold color foundin films of gold, but rather a wine red hue that Faraday correctlyattributed to a high state of dispersion and a small size heestimated in the 10 nm range. Modified versions of his syntheticapproach are still utilized in modern studies.3-5

The Faraday synthesis as modified by Turkevich3 is the basisof most solution-based formation of nanoparticles of gold andsilver in water or polar solvents. These colloids are stabilizedby surface charge, typically in the form of citrate ion.3-7 Suchmethods usually produce small populations of triangular androd-like structures in addition to spherical ones. Because theoptical properties of metal nanoclusters are very sensitive toshape as well as to surface charge, these synthetic drawbackscomplicate detailed theoretical analysis of the size-dependentoptical properties of these nanoparticles. Also, it is difficult tosynthesize metal clusters in aqueous solution with dimensionsless than 5 nm. Nanoparticles of Ag and Au less than 5 nm are

particularly interesting as they may exhibit both classical andquantum size effects in their absorbance spectra.1,6,7

Nanoalloys of gold and silver, metals that have essentiallyidentical lattice constants and are completely miscible, presentnew opportunities to investigate the effect of nanostructure onoptical properties. We expect their optical properties mightdepend not only on size, shape, and composition, but also onwhether the silver and gold is randomly distributed throughoutthe nanoparticle or segregated into a core/shell nanostructure.For small nanoparticles with a size less than 5 nm, asinvestigated in the present study, this structural distinction canbe very challenging to elucidate based upon conventionalelectron microscopy, so the optical properties may provide thebest way to distinguish nanoalloys from core/shell nanoalloys.However, to accomplish this goal requires a synthesis andselection technique to identify particles of identical shape andsize, making nanostructure and composition the only significantvariables.

Two nanostructures of AgAu nanoparticles have been studiedpreviously, core/shell structures that require a sequential reduc-tion process and nanoalloys that are typically synthesized bysimultaneous coreduction of precursor metal salts.8-20 Nano-structures with a Ag core and Au shell, Ag/Au, were synthesizedusing Ag seed particles onto which Au was reduced anddeposited either radiolytically8 or chemically.16,18 Other studiesinvestigated Au/Ag nanostructures, made by depositing Ag onAu core particles.12,13,15,17,19 Finally, by coreduction of bothmetals, a nanoalloy type structure was investigated.9-11,14 A fewstudies investigated both nanoalloy and core-shell nano-stuctures15,18,20 Even when one metal is deposited onto the seedsfrom another, it is often observed that interdiffusion of the shellatoms into the core or vice-versa to form a diffuse interfaceoccurs.9 This typically occurs when a more noble metal suchas Au is deposited onto a less noble one like Ag.11 When thisoccurs, the optical absorbance will change with time aftersynthesis as the nanostructure evolves. This structural evolutionmight involve annealing of defects and/or interdiffusion ofsegregated atoms. Because the time between synthesis andmeasurement of optical properties is rarely considered animportant variable, comparison of results from various groupsis complicated.

† Part of the “J. Michael Schurr Special Section”.* E-mail: jpwilco@gmail.com.

J. Phys. Chem. B 2009, 113, 2647–2656 2647

10.1021/jp806930t CCC: $40.75 2009 American Chemical SocietyPublished on Web 12/19/2008

In the case of both large, D > 5 nm, nanoalloys, chargestabilized in water,10,13 and small, 2-3 nm nanoalloys, stericallystabilized by ligands such as alkanethiols in oils like toluene,9,11,15

only a single absorbance peak is usually observed in the opticalabsorbance spectra, whose position shifts continuously from thatof pure Ag clusters, (∼420 nm), to that of pure Au clusters,(∼520 nm) as a function of composition. Interestingly, evenfor the case of core/shell nanostructures such as Ag/Au8,9,11,16

and Au/Ag,13,15 only a single plasmon absorbance peak isobserved for clusters for sizes less than ∼12 nm, although simpleMie theory predicts that two peaks should be observed. Foreither charge or sterically stabilized clusters, the deposition ofeven small, monolayer amounts of Au onto Ag clusters causesvery large broadening of the initially sharp Ag plasmonabsorption followed by very significant red shifting of theresulting asymmetrical peak, just in the nanoalloy case. Thisobservation is often rationalized by invoking an alloying effectin the shell or at the interface between Ag/Au.8,12,15,18 However,pure Ag or Au clusters with sizes of only 2-3 nm have suchbroad absorbance plasmons that it is doubtful whether one wouldbe able to observe two distinct absorbance peaks even with asharp interface between the metals. It is very difficult to verifythis surface alloying effect by structural measurements such aselectron microscopy since the lattice constant of Au and Agare essentially identical.17 Recent high angle annular dark fieldmicroscopy studies of bimetallic Ag/Au and Au/Ag core/shellnanoparticles have shown that the core is predominantly Ag,and no evidence of interatomic diffusion at the interface couldbe observed.20 Thermodynamic considerations suggest that Agwould prefer to be in the shell of these nanostructures, but thesecalculations do not account for the metal ligand binding at thesurface of surfactant stabilized clusters, and this favors thepresence of Au at the surface.

In this paper we describe the formation of nanosize, neutralbimetallic particles in low dielectric constant, nonpolar mediausing inverse micelles as reactors to solubilize ionic saltprecursors and then reduce them chemically to form seedparticles of both Ag and Au.21 A heterogeneous growth methoddescribed previously is then used to form core/shell, Ag/Au andAu/Ag particles, whereas coreduction of Ag and Au salts isused to form AgAu nanoalloys.24 These uncharged sphericalnanoparticles are sterically stabilized against aggregation by thesurfactant used to form the inverse micelle. Upon addition ofstrongly binding ligands, such as alkyl thiols, it is possible to

size-select and analyze their optical properties using analyticalmethods like size-exclusion chromatography, SEC.7,22,23 Nano-particles prepared and analyzed by SEC can be studied inidentical chemical environments, and SEC will separate nano-particles with differing shape (e.g., rod-like, triangular) as well.

We investigate the effect of Ag/Au ratio on the peak plasmonabsorbance wavelength and the damping or absorbance linewidth of the absorbance for both alloy and core/shell nano-structures in identical chemical environments. We show thatincreasing the Au content of Ag/Au core/shell nanoparticlesleads to a red shift and increased damping of the plasmonwhereas an increase in Ag content in Au/Ag nanoparticles resultsin a blue shift and reduced damping. The nanoalloys also red-shift with increasing Au content and exhibit increased linebroadening (i.e., energy loss or damping) for smaller sizes. Fora constant composition, each type of nanostructure can bedistinguished and identified solely by its optical absorbancefeatures. The ability to control size, composition, and nano-structure to create a unique absorbance signature should beuseful in applications where the nanoparticles serve as chemicallabels or taggants.

II. Experimental Section

Nanocluster Synthesis. Our synthesis of core/shell nano-particles uses a heterogeneous seed growth technique that wehave previously described in detail.24 In this process a solutionof purified seed nanoparticles that forms the nanoparticle coreis first synthesized using the inverse micelle nanoparticle growthmethod.5,6,21 The size and size dispersion of these seeds isverified using size-exclusion chromatography as described belowand in previous publications.7,22,23,25 The commercially availablemetal salt precursors and concentrations of each inverse micellesolution used to generate the Ag, and Au seed particles andthen deposit either Au or Ag on their surface to form Ag/Au orAu/Ag nanoparticles are shown in Table 1. The samples arereferenced in the text and tables by their SEC diameter, d, andratio of Ag/Au. For example, Ag/Au, 1/2, d ) 4.8 nm is a core/shell particle with a Ag core/Au shell nanostructure, a composi-tion of 1 atom Ag for 2 atoms of Au (66% Au), and a totalparticle diameter of 4.8 nm. Table 2 shows the diameters, d, asmeasured from size exclusion chromatography, SEC andtransmission electron microscopy, TEM. Table 2 also sum-marizes optical absorbance properties such as the peak absor-

TABLE 1: Synthesis Conditions

sample metal salt(s) surfactant reductant stabilizer solvent

Ag, d ) 4.0 AgBF4 TOAC (0.1 M)2 LiBH4 (0.04 M) C12H25SH (0.01 M) tolueneAg, d ) 3.2 AgNO3 TOAC (0.1 M) LiBH4 (0.04 M) C12H25SH (0.01 M) tolueneAg, d ) 2.4 Ag(C6H4CO2) None NaBH4 (0.1 M) C12H25SH (0.01 M) tolueneAu, d ) 2.0 HAuCl4 TOPB (0.1 M)3 LiAlH4 (0.04M) C12H25SH (0.01 M) benzeneAu, d ) 4.0 HAuCl4 C12E5 (0.2 M)4 Li(C2H5)3BH (0.04 M) C12H25SH (0.01 M) octaneAg/Au, 1/1, d ) 4.4 AuPPh3Cl1 N.A. NaBH4 (0.1 M) C12H25SH (0.01 M) tolueneAg/Au, 1/2, d ) 3.1 AuPPh3Cl N.A. NaBH4 (0.1 M) C12H25SH (0.01 M) tolueneAg/Au, 1/2, d ) 4.8 AuPPh3Cl N.A. NaBH4 (0.1 M) C12H25SH (0.01 M) benzeneAu/Ag, 1/1, d ) 3.5 Ag(C6H4CO2) N.A. NaBH4 (0.1 M) C12H25SH (0.01 M) benzeneAu/Ag, 1/1.9, d ) 3.5 Ag(C6H4CO2) N.A. NaBH4 (0.1 M) C12H25SH (0.01 M) benzeneAu/Ag, 1.9/1, d ) 3.1 Ag(C6H4CO2) N.A. NaBH4 (0.1 M) C12H25SH (0.01 M) benzeneAgAu, 2/1, d ) 4.5 HAuCl4, AgBF4 TOAC (0.1 M) LiBH4 (0.04 M) See text tolueneAgAu, 1/1, d ) 5.0 HAuCl4, AgBF4 TOAC (0.1 M) LiBH4 (0.04 M) See text tolueneAgAu, 1/2, d ) 4.3 HAuCl4, AgBF4 TOAC (0.1 M) LiBH4 (0.04 M) See text tolueneAgAu, 2/1, d ) 3.2 HAuCl4, AgBF4 TOAC (0.1 M) LiBH4 (0.04 M) C12H25SH (0.01 M) tolueneAgAu, 1/1, d ) 3.1 HAuCl4, AgBF4 TOAC (0.1 M) LiBH4 (0.04 M) C12H25SH (0.01 M) toluene

1 AuPPh3Cl ) gold triphenylphosphine chloride ) (C6H5)3PAuCl. 2 TOAC ) tetraoctylammonium chloride ) (CH3(CH2)7)4NCl. 3 TOPB )tetraoctylphosphosium bromide ) (CH3(CH2)7)4PBr. 4 C12E5 ) penta-ethyleneglycol-mono-n-dodecyl ether.

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bance wavelength, λp, and the half-width at half-height to thered of this peak, ∆λ1/2. In most cases, the absorbance to theblue of the asymmetrical absorbance peak is too large to allowdetermination of the half-width to the blue of the peak.

The cationic surfactants listed in the footnotes of Table 1,tetraoctylammonium chloride (TOAC) and tetraoctylphospho-sium bromide (TOPB), were purchased from Fluka chemicals.The nonionic surfactant, penta-ethyleneglycol-mono-n-dodecylether (C12E5) was obtained in highly purified form from NikkoChemicals, Japan. The metal salts, HAuCl4 and AgBF4; metal-organics, gold triphenylphosphine chloride, (C6H5)3PAuCl, andsilver benzoate, Ag(C6H4CO2); and solvents, toluene andbenzene, were purchased from Aldrich chemical and used asreceived.

A feed stock source of Ag or Au atoms that is easily reducedby either LiBH4 or NaBH4 and is soluble in the same solventas the seed nanoparticles is required for the heterogeneousgrowth approach.24 We have determined optimal organometallicsources to be gold triphenylphospine chloride and silverbenzoate, since both of these metal-organics are soluble inbenzene or toluene. In each synthesis we select a core particlewith a size that will result in a core/shell nanoparticle with adesired Ag/Au ratio and final size. However, in certain cases,the final size and Ag/Au ratio differs from that calculated dueto incomplete deposition. We now provide a description of aspecific synthesis of each nanostructure with a final size of∼4-5 nm and a 1:1 Ag/Au ratio.

Ag and Au Seed Nanoparticles. Following the heteroge-neous growth method previously described,24 we prepared Agcore particles with sizes of 2.4, 3.2, and 4.0 nm. These pureAg nanoparticle solutions also provide a reference for com-parison of optical absorbance to that of Ag/Au and AgAunanostructures. An inert gas glovebox was used due to the airsensitivity of the reducing agents employed. For example, (seeTable 1), a 2.4 nm Ag nanoparticle seed solution was grownby NaBH4 reduction of 0.01 M silver benzoate in toluenecontaining the stabilizer dodecanethiol. Larger, 4.0 nm seed Agparticles solutions were formed by LiBH4 reduction of 0.01 MAgBF4 in an inverse micelle solution of the cationic surfactant,TOAC (0.1 M) in toluene. AgBF4 dissolved completely, forminga transparent solution, following magnetic stirring for severalhours. Dodecanethiol at 0.01 M was added prior to reductionby a 1 M stock solution LiBH4 dissolved in THF. Vigorousevolution of hydrogen and formation of a dark yellow/redsolution occurs rapidly, less than 1 min after addition of thereductant.

Similarly, 0.01 M of HAuCl4 was dissolved via magneticagitation in an inverse micelle solution of TOPB (0.1 M) intoluene. Upon addition of the stock 1 M solution LiAlH4 inTHF to a final concentration of 0.04 M, vigorous bubbling ofH2 is observed and a change in color to dark orange/red occurredin less than a minute, producing 1.8-2 nm clusters for use asseeds for the Ag/Au nanostructures described in Tables 1 and2. Deposition of additional gold atoms as described previously24

was used to grow larger Au particles for use as seeds to growAu/Ag nanostructures. Au, d ) 4 nm seeds could also be growndirectly using the recipe of Table 1.

Ag/Au Nanoparticles. A 4 mL, 0.01 M solution of Ag, d )3.2 nm core particles in benzene containing 0.002 M of thealkyl thiol, dodecanethiol, and a reducing agent, NaBH4 (0.1M,final concentration), is placed in a vial containing a magneticstir bar. The vial has a septum-sealed cap, and the sample isprepared and sealed under argon. A gas-tight syringe is filledwith a 0.01 M solution of AuPPh3Cl in benzene and connectedto the vial containing the seed particles with a 1/16” I.D. Teflontube terminated in a needle that penetrates the septum of thevial. A syringe pump delivers the AuPPh3Cl in benzene solutioninto the magnetically stirred vial at a rate of 4 mL/hr. Reductionof the Au(I) by NaBH4 and release of hydrogen gas is observed.A solution color change from yellow to reddish/yellow alsooccurs as Au is deposited onto the 3.2 nm Ag seeds. After 4mL is delivered into the solution, a 1:1 Ag:Au nanoparticleforms, sample Ag/Au, 1/1, d ) 4.4 nm, Table 1.

Au/Ag Nanoparticles. A series of Au core particles with sizesof 2, 2.4, 2.8, 3.2, and 4.0 nm are prepared as describedpreviously.24 Two specific recipes for Au, d ) 2 nm and Au, d) 4.0 nm seeds are shown in Table 1. A 4 mL solution of 0.01M solution of 2.4 nm Au core particles in benzene or toluenecontaining 0.002 M of the alkyl thiol dodecanethiol, and areducing agent, 0.1 M NaBH4, is placed in a vial containing amagnetic stir bar. The vial has a septum-sealed cap and thesample is prepared and sealed under argon. A gas-tight syringeis filled with a 0.01 M solution of silver benzoate, Ag(C6H4CO2),in benzene and connected to the vial containing the seed particleswith a 1/16” I.D. Teflon tube terminated in a needle thatpenetrates the septum of the vial. A syringe pump delivers the0.01 M Ag(C6H4CO2) feedstock solution into the magneticallystirred vial at a rate of 4 mL/hr. After 4 mL is delivered intothe 0.01 M Au seed solution, a 1:1 Ag:Au nanoparticle isproduced, Au/Ag, 1/1, d ) 3.5 nm, Table 1.

AgAu Nanoalloys. An inverse micelle solution containing0.01 M HAuCl4 and 0.01 M AgBF4 is formed by dissolving

TABLE 2: Cluster Size and Optical Properties

sample te (min) d (nm, SEC) d (nm, TEM) λp (nm) ∆λ1/2 (nm)

Ag, d ) 4.0 7.16 4.0 4.3 449 59Ag, d ) 3.2 7.37 3.2 N.A. 460 75Ag, d ) 2.4 7.62 2.4 2.5 493 96Au, d ) 2.0 7.7 2.0 2.0 none N.A.Au, d ) 4.0 7.15 4.0 4.2 513 91Ag/Au, 1/1, d ) 4.4 7.08 4.4 4.6 501 74Ag/Au, 1/2, d ) 3.1 7.41 3.1 3.4 511 111Ag/Au, 1/2, d ) 4.8 6.99 4.8 5.0 491 98Au/Ag, 1/1, d ) 3.5 7.3 3.5 3.7 511 82Au/Ag, 1/1.9, d ) 3.5 7.28 3.5 N.A. 487 78Au/Ag, 1.9/1, d ) 3.1 7.41 3.1 3.2 501 103AgAu, 2/1, d ) 5.0 6.92 5.0 na 487 59AgAu, 1/1, d ) 4.8 6.97 4.8 5.5 492 62AgAu, 1/2, d ) 4.3 7.13 4.3 4.5 511 76AgAu, 2/1, d ) 3.0 7.45 3.0 3.2 491 90AgAu, 1/1, d ) 3.2 7.4 3.2 3.7 506 82

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these metal salts in a 0.1 M solution of the cationic surfactanttetraoctylammonium chloride (TOAC) in toluene using vigorousstirring overnight in a glovebox. The transparent precursorsolution is placed in a vial with a magnetic stir bar, and a stocksolution of 1 M LiBH4 in tetrahydrofuran (THF) is rapidlyinjected, resulting in a final reducing agent concentration of0.04M. The LiBH4 reductant rapidly reduces the light yellowprecursor solution to a dark orange/red color with the evolutionof hydrogen gas bubbles. The reduction is done in a gloveboxbecause of the very reactive and moisture-sensitive nature ofthis reducing agent. The nanoalloys are not air sensitive andcan be handled outside the glovebox. The ratio of the Au andAg precursor salts is used to change the final Ag/Au ratio inthe nanoparticles. The alloy nanoparticles studied range in sizefrom 3.1 to 5.5 nm. An example is AgAu, 1/1, d ) 5.0 nm inTable 1. To permit purification and chromatographic analysisof the alloy clusters, an alkyl thiol is added a day after reduction.The smaller 3 nm AgAu shown are made in an identical method,but the alkyl thiol is added prior to reduction.

By-product salt and excess surfactant were removed byaddition of a 10-fold volumetric excess of either methanol oracetone to the solution. These nonsolvents result in precipitationof the alkyl thiol passivated alloy nanoclusters from the solution.Mild centrifugation for 30-60 min at 1000g compacts theprecipitate and allows the supernatant to be decanted and thenanoalloys washed in more nonsolvent and then redissolved inthe solvent, toluene, used for chromatographic, TEM, and XRFanalysis.

Size-exclusion Chromatography (SEC). We use a com-mercial Water’s Corporation Delta-prep high pressure liquidchromatograph to size fractionate and study the absorbanceproperties of the alloy nanoparticles. The system consists of anautosampler (model 717), used to inject 10 µL of the 0.01 Mnanoparticle solutions into a mobile phase; a solvent pump thatdelivers a steady flow of the mobile phase, toluene, through adegasser; a solvent filter; a guard column; and a SEC column.

The clusters are separated by size and shape using a PolymerLaboratories type PL1000 column with dimensions of 7.8 mm(diameter) × 250 mm (length) packed with 1000 Å pore sizepolystyrene microgel particles with an average size of 5 µm.This high resolution column yields an elution peak full widthat half-height of 0.25 min, which determines our ability toresolve separate peaks corresponding to clusters of different size.

The elution of the nanoparticles and nonabsorbing chemicalsis detected online using a photodiode array, PDA, (model 996)and a differential refractive index detector (model 410) tomonitor nonabsorbing chemicals such as surfactants. A mobilephase flow of 1.0 mL/min was used that, for the column porevolume of the PL1000 column, means that clusters elute between5 and 12 min, corresponding to a hydrodynamic size range of1-10 nm. Further details regarding the size calibration and otherinstrumental details were given previously.22,23 As in previouswork, a mobile phase additive of dodecane thiol at 0.01 M intoluene was used to minimize chemical interactions betweenthe clusters and the column. We use the 400 nm wavelengthchannel of the PDA to detect the visible absorbance associatedwith elution of the nanoparticles. The complete absorbancespectra from 290 to 800 at 4 nm bandwidth is also collectedevery 2 s. This corresponds to about 15 complete spectra in aresolution-limited 30 s elution peak.

In previous work we demonstrated that the elution time te isrelated to the cluster hydrodynamic diameter, Dh by te ≈ logDh. A best-fit to a series of linear alkanes and polystyrenepolymer standards was used to obtain a calibration of Dh as

measured by dynamic light scattering with te. The metalnanocluster core diameter is obtained from Dh by subtractingthe organic passivating thickness. This thickness can beestimated by addition of a several different alkyl thiols to acluster solution and measuring the value of Dh. Using the coresize from TEM then yields the organic passivating thickness.For dodecanethiol used in the present study the organicpassivating thickness was found to be 1.2 ( 0.1 nm.22,24,25

Transmission Electron Microscopy (TEM). In these studiestransmission electron microscopy was used primarily to confirmthe hydrodynamic size obtained by SEC and to calibrate thecolumn using a series of Au cluster size standards. The TEMsamples were prepared as in our previous work by depositingabout 2 µL of a 0.01 M nanoalloy solution onto a holey carbongrid resting on a piece of filter paper. The wicking action ofthe filter paper rapidly draws the solvent through the holes inthe grid and spreads the clusters over a large region of the grid,avoiding cluster pile up during the drying process. Since theclusters have an alkyl thiol on their surface regions, hexagonallypacked nanoparticles often form as shown in Figures 3, 8and 9.

X-ray Fluorescence (XRF). The Au and Ag concentrationsin the purified core/shell and alloy nanoparticles that wereanalyzed using a QuantXTM XRF instrument with a Rh X-raytube providing X-rays with energies from 1-45 KeV andthermoelectrically cooled X-ray detector with an energy resolu-tion of 0.25 KeV. The response was calibrated using knownconcentrations of stock solutions of AuPPh3Cl and Ag(C6H4CO2)in benzene. The stock solutions were mixed in ratios appropriatefor the Ag:Au ratios investigated in the samples. The Ag:Auratios listed in Tables 1 and 2 were obtained by comparison ofthe Ag (K line at 22.104 KeV) and Au (L line at 9.711 KeV)XRF peak areas from the core/shell and alloy nanoparticles tothe standard samples.

III. Results and Discussion

Figure 1 shows the absorbance signal at 400 nm from theonline PDA spectrometer vs elution time for three Ag/Au metalnanocluster solutions described in Tables 1 and 2. Each sampleis stabilized with an organic shell of dodecanethiol. The totalthickness of the organic shell has been determined previously

Figure 1. Chromatograms, solid lines, showing absorbance at 400 nmvs elution time for three Ag/Au samples. Open circles label the Ag, d) 2.4 nm seed, green curve; open squares are the Ag/Au, 1/2, d ) 3.1nm, blue curve; and open triangles are the Ag/Au, 1/2, d ) 4.8 nm,red curve.

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for Au nanoclusters and is assumed to contribute 2 × 1.2 )2.4 nm to the measured hydrodynamic diameter of the nanoal-loys as well. This total shell thickness was subtracted from thehydrodynamic diameter as measured by SEC to obtain the corediameter sizes indicated in this figure and Table 2. Theabsorbance chromatograms of Figure 1 show that the smallestAg clusters elute at a time that corresponds to a metal corediameter of 2.4 ( 0.2 nm. The smallest clusters spend thegreatest amount of time exploring the pore structure of the SECcolumn and thus have the longest retention time of the threesamples shown. These Ag, d ) 2.4 nm clusters are used asseeds to grow the Ag/Au, 1/2, d ) 3.1 nm cluster sample whosechromatogram is also shown in this figure. The larger size ofthe resulting core/shell particles gives a single elution peakwhose faster elution corresponds to a core size of 3.1 ( 0.3nm. Furthermore, the elution line width, or size dispersion, is

as good as the seeds. The elution line width of 0.25-0.3 min isvery close to that of a monodisperse molecule of a solvent likeoctane, and indicates negligible size (and shape) dispersion inthese two samples. No peak at the original seed elution time isobserved, demonstrating that the Au is deposited entirely ontothe seeds.

If a larger 3.2 nm Ag seed is used, and the same amount ofAu is deposited to form a Ag/Au nanostructure, then theresulting Ag/Au, 1/2, d ) 4.8 nm sample elutes at the shortesttime as expected for a SEC separation mechanism. The elutionpeak of this sample shown in Figure 1 corresponds to a size of4.8 ( 0.4 nm, in good agreement with the core size of 5 ( 0.5nm estimated from the TEM shown in Figure 3. The orderingof the particles in this TEM shows that the dodecanethiol ligandis still present on the nanoparticle surface following the Audeposition. The line width of the elution peak for this Ag/Au,1/2, d ) 4.8 sample is larger than the other two samples shown,

Figure 2. Bright-field TEM of a hexagonally ordered region of theAg/Au, 1/2, d ) 4.8 nm sample whose chromatogram and absorbanceis shown in Figures 1 and 4.

Figure 3. Absorbance spectra collected online during the chromatog-raphy of Ag/Au, 1/2, d ) 4.8 nm clusters for the two elution timesgiven in the legend.

Figure 4. Normalized absorbance spectra obtained at the elution peaksin figure 1 vs wavelength. Open circles, red curve is a reference Ag, d) 4 nm spectrum; open triangles, green curve, is the Ag/Au,1/2, d )3.1 nm sample; open squares, blue curve, is the Ag/Au, 1/2, d ) 4.8nm sample; and filled circles, black curve, is a reference Au, d ) 4.0nm, absorbance spectrum.

Figure 5. Absorbance spectra normalized at their peak, collected atthe peak elution times noted in Table 2 for the following samples: Ag/Au, 1/2, d ) 4.8, open circles, red curve; Ag/Au, 1/2, d ) 3.1, opensquares, blue curve; Au, d ) 4.3, open triangles, green curve; and Ag,d ) 4.0, solid circles, black curve.

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which may indicate more size and/or shape polydispersitycompared to the other two samples.

A small elution peak near t ) 8.6 min in Figure 1 couldindicate the formation of a small population of very small Auclusters, and this long retention time corresponds to a size ofonly ∼1.0 nm. However, if one compares the spectra at themajority peak at t ) 7.0 min with that at t ) 8.6 min, Figure 4,one observes no significant difference in peak absorbancewavelength or line width. This means that the cluster subpopu-lation eluting at this delayed time must interact chemically withthe column material instead of following a strict size exclusionmechanism. So, there are two distinct surface structures withnearly identical average size.

In previous work we have shown how one can learn aboutthe strength of binding of the passivating ligand to a cluster ofa given size and composition.25 In an ideal SEC elution processthere should be no specific chemical interaction of the metalcluster with the column. We try to achieve this by passivationof the cluster surface with alkyl thiols. If complete passivationis successful, then one should observe a nearly symmetricalelution profile with the excess line width, (excess line width )observed line width-instrumental line width), determined solelyby the sample polydispersity. This line width can be as smallas observed in a pure solvent for the best Au preparations.22

However, if the stabilizing cluster ligand binds only weakly tothe cluster surface, then a significant fraction of the injectedclusters may bind to the column and fail to elute. This can bequantified by removing the column and measuring the elutionpeak area using the same detector conditions and comparing itto the area when the column is used for size fractionation. Forpure Au clusters stabilized by alkyl thiols, the ratio of theseareas, a measure of the SEC column elution efficiency, is 100%within experimental error. For Ag clusters it is often lower,40-80%, showing the binding to be weaker between Ag anddodecanethiol. Not surprisingly, coating the Ag seeds with Auincreases this efficiency, showing that the presence of Au atomsat the cluster surface enhances thiol binding, whereas coatingAu clusters with Ag results in lower column efficiency and moreelution peak asymmetry.

The elution time at the peak of the chromatogram shown inFigure 1 represents the average size of the clusters from thatsample, and the absorbance spectrum collected at this timerepresents the average absorbance profile. Figure 5 shows thepeak elution absorbance spectra from two sizes of the Ag/Au,1/2 samples. For comparison, absorbance spectra from 4 nmpure Ag and Au clusters are also shown. The effect of the Aushell on the absorbance is to blue shift the peak relative to apure Ag cluster of the same size while broadening the absor-bance line width. The optical absorbance line width is similarto that of the pure Au reference sample and is quite asymmetricaround the peak. As in the case of pure Ag clusters,7 decreasingthe Ag/Au nanoparticle size from 4.8 to 3.1 nm red shifts theabsorbance peak. So, the red shift of the peak with decreasingsize for Ag/Au nanoparticles indicates that the Ag core controlsthe direction of the peak shift, since pure Au clusters blue shiftwith decreasing size. Meanwhile, the Au shell leads to majorbroadening of the peak. This shows that the interband transitionsof the Au shell, which dominates the higher energy, shorterwavelength region of the absorbance is a strong component ofthe total absorbance oscillator strength. This is the reason whythe symmetry of the pure Ag absorbance peak is lost in theAg/Au structures. The long wavelength absorbance line widthof the Ag/Au samples is quite similar to the pure Au, d ) 4.0nm cluster sample shown. So, the energy loss and damping

mechanism reflected in the line width is similar to pure Aunanoclusters. This suggests that the energy loss due to inelasticscattering at the Au/solvent interface is more important thanthe loss at the Ag/Au interface. However, evidence for Ag inthe core is still reflected in the weaker absorbance to the blueof the peak for both Ag/Au nanostructures compared to the pureAu, d ) 4 nm reference spectra. If more Au is deposited ontothe Ag core, then a small additional red shift from 520 to 530nm is observed. However, the most significant peak red shiftand broadening occurs with small amounts of deposited Au ashas been noted in the earliest work in this area.8

As can be observed in Figure 5 and Table 2, Ag/Aunanoparticles with an average total size of around 4-5 nm haveabsorbance spectra that red-shift and broaden with increasingamount of deposited Au. Indeed, the Ag/Au, 1/2, d ) 4.8nanoparticle solution has a spectrum nearly identical to a pureAu nanocluster of the same size (see Table 2). The symmetryof the absorbance profile for 4 nm size Ag nanoparticles aroundthe peak absorbance position is lost rapidly as Au is depositedon the surface, indicating the absorbance mechanism at shorterwavelengths is strongly influenced by the interband transitionsfrom d-type orbitals to sp-type conduction band of the Au onthe cluster surface.

The observation of a red shift and increase in line width withincreasing Au content for Ag/Au nanostructures is common tosamples prepared in both aqueous and nonaqueous solution. Inthe first study of Ag/Au nanostructured particles by Mulvaneyand co-workers, 7.6 nm Ag nanoparticles were used as seedsfor radiolytically reduction and deposition of Au.8 As noted inthe original work, one might expect these larger particles, basedupon Mie theory, to exhibit two absorbance peaks correspondingto the Ag and Au plasmon resonance energies if a sharp interfacebetween Ag core and Au shell is maintained during the growthprocess. As in the present experiments, only a small amount ofAu deposited on the Ag nanoparticle, less than a monolayer,was sufficient to cause a broadening and disappearance of thesharp, symmetrical Ag plasmon absorbance. This rapid dampingof the Ag absorbance was attributed to formation of a surfacealloy of AgAu due to diffusion of the Au atoms into thenanoparticles. For smaller clusters, as in the present work,broadening of the absorbance spectra simply due to increasedelectron scattering at the cluster/solvent interface could alsoobscure the distinct absorbance peaks from Ag and Au.

A similar spontaneous alloying attributed to interdiffusionof Ag and Au was reported by Shibata and co-workers for Au/Ag nanostructured particles.12 However, later measurements byChen and co-workers using X-ray absorption fine structure,XAFS, to study both nanoalloys and Ag/Au nanostructures couldnot distinguish the two types of structure based upon the opticalspectra alone.18 In both cases, 10 nm nanostructured particlesshowed a red-shift and broadening occurs with increasing Aucontent. XAFS did show distinct core/shell and alloy nano-structures depending on the synthesis method.

Because the Mulvaney experiments took place in aqueoussolution and the particles were likely charge stabilized, theirobservations could reasonably differ from the present study.Later observations on Ag/Au nanostructures by Shon and co-workers are more directly comparable to the present work sincethe nanoparticles were sterically stabilized, prepared, and studiedin nonpolar solvents.9,11 In these experiments, gold was depositedonto preformed 4.2 nm Ag nanoparticles stabilized by dode-canethiol. The plasmon absorbance from the Ag core wasbroadened and red-shifted as more gold was deposited. A mostinteresting observation was that Au was incorporated into the

2652 J. Phys. Chem. B, Vol. 113, No. 9, 2009 Wilcoxon

core via diffusion from the surface. In fact, X-ray photoelectronspectroscopy9 (XPS) showed that for Ag/Au, 2.5/1 nanoparticlesabout 60% of the surface atoms were Ag whereas in Ag/Au,1/2.5, over 86% of the surface sites were Ag. This result wouldnot be expected based upon the stronger binding of Au withdodecanethiol compared to Ag, which would favor Au at thesurface.7 However, not all the cluster surface atoms are ligatedto the dodecanethiol, so migration of the unbound atoms intothe interior is a possible explanation. The fraction of Ag andAu at the nanoparticle surface was inferred from the relativeXPS S2p doublet peaks areas whose binding energies wereknown to correspond to Au-thiol or Ag-thiol surface sites.XPS measurements are a global composition average over theentire sample and thus cannot give the composition of anindividual 4.0 nm nanoparticle.

Another a surprising observation in these studies11 was anapparent shrinkage of the final nanoalloy particle size from seed,Ag, d ) 4.2, to 3.0 nm. A similar shrinkage of the Ag core wasnoted by J. Yang and co-workers when forming Ag/Aunanoparticles in toluene.16 A possible explanation of this coreshrinkage is that some of the Au was deposited on the 2 nmAg seed subpopulation observed in their electron microscopyand that these smaller nanoparticles grew at the expense of thelarge 4.2 nm Ag seeds via a etching process where the largernanoparticles lose surface atoms to the smaller clusters in theinitially bimodal Ag nanoparticle distribution.

Etching and aging of Au nanoparticles in the presence of alkylthiols has been established by previous work.25 This processcan narrow the size distribution as well as leading to a smalleraverage size. The time scale for the etching depends on the chainlength of the alkyl thiol, being faster for shorter chain thiolsbut requiring many weeks for dodecanethiol, the thiol used inthe present work. Because our optical measurements and SECwere obtained only a few days postsynthesis, we do not believeongoing redistribution of atoms between clusters plays asignificant role in the optical properties reported. Figure 2, aTEM of the Ag/Au, 1/2, d ) 4.8 sample confirms that sizeestimated by the chromatography is approximately 5 ( 0.5 nmand that no major etching of the core/shell nanoparticles hasoccurred. The lighter contrast of the lower Z Ag core of manyof the particles shown is consistent, but does prove a Ag/Aunanostructure since particle orientation on the grid affectsdiffraction contrast as well. We have previously examined thenanostructure of Ag/Au, 1/2 and Ag/Au, 2/1 nanoparticles witha total size of around 4 nm using high angular dark field imagingmicroscopy, HAADF.17 By modeling the HAADF intensity lineprofiles through the Ag/Au nanoparticles and comparing toexperimental data, it was established that Au atoms wereconcentrated in the shell and that Ag atoms were mainly in thecore for the 1:2 composition of Figure 4. However, interdiffusionat the interface could not be ruled out, especially for the 2/1composition. This is likely to remain a difficult issue to settleby structural methods alone because of the high spatial resolutionrequired.

If interdiffusion of atoms at the Ag/Au or Au/Ag interfacedestroys the core shell structure, than we expect the opticalabsorbance of all three nanostructures to be identical. In Figure6 we show absorbance profiles normalized to their peaksobtained during SEC of core/shell and alloy nanoparticles withsizes ranging from 3.5 to 4.8 nm. The absorbance profile ofeach has a unique shape and position depending on whether asequential or coreduction was used to form the clusters. Forexample, AgAu, 1/1, d ) 4.8 alloy nanoclusters have a blue-shifted and narrower peak compared to either Ag/Au, 1/1 or

Au/Ag, 1/1 samples. Comparing the Ag/Au, 1/1 to the Au/Ag,1/1 spectra, we observe that the Ag/Au sample is slightly blue-shifted compared to the Au/Ag sample. The broadening to thered of the peak is the same within experimental error, but thepresence of Au in the shell of the Ag/Au sample results in amore asymmetrical absorbance profile, with stronger shortwavelength absorbance, nearly identical to a pure Au sampleof the same size. This shows that most of the energy dissipationdue to electron scattering is occurring at the shell/solventinterface. The more random distribution of Ag and Au atomsin the AgAu, 1/1 nanoparticle sample produces a spectrum thatdiffers significantly from a pure Ag or Au sample of the samesize (see Figure 5) and whose peak is blue-shifted and narrowerthan either the Ag/Au or the Au/Ag nanostructure.

The results shown in Figure 6 demonstrate that even if onewere to analyze each of these samples for average size andcomposition using TEM and XRF one would still not be ableto predict the optical absorbance of a dispersion of thesenanoparticles. The absorbance depends on the distribution ofatoms within the cluster, determined by the sequence ofreduction of the two metals. However, the data of Figure 6allows one to prepare samples having unique optical tags andsubsequently identify any sample labeled with a chosen tag fromits optical absorbance profile alone.

Three Au/Ag samples with compositions of 1.9/1, 1/1, and1/1.9 were investigated. The optical properties of these samplesare shown in Table 2. The Au/Ag, 1.9/1, d ) 3.1 nm sampleshows very little effect of the Ag deposited on the Au on theabsorbance peak at 501 nm or the line width. The presence ofthe Ag shell does not affect the optical properties to the samedegree as the deposition of Au on Ag nanoparticles. As theamount of Ag deposited increases to 1/1, the line width reflectingthe amount of damping of the electron oscillation in the Au/Agnanoparticle is reduced. As the amount of Ag is increased furtherto 1/2, a blue shift in the peak position is finally observed witha small decrease in line width. Comparing this behavior to theAg/Au samples in Table 2, the relative effect of the Ag/Au ratioon both line width and peak position is smaller for the Au/Agnanostructures.

The mild changes in peak position and line width we observedin 4 nm Au/Ag nanoparticles differ from that reported for 12

Figure 6. The effect of nanoparticle structure on absorbance is shownby absorbance spectra of three samples of nearly similar compositionbut different spatial distribution of Ag and Au atoms: Ag/Au, 1/1, d )4.4, open circles, red curve; Au/Ag, 1/1, d ) 3.5, open squares, bluecurve; and AgAu, 1/1, d ) 4.8, open triangles, green curve. Spectrawere collected online at the elution peak during chromatography.

Dispersed Gold and Silver Alloy Nanoparticles J. Phys. Chem. B, Vol. 113, No. 9, 2009 2653

nm Au seed nanoparticles with Ag deposited on the surface byLu and co-workers.13 That study reported a large shift in thepeak absorbance from 520 nm (12 nm gold seed) to 420-430nm (27 nm Au/Ag nanoparticle) with significant narrowing ofthe line width and a much more symmetrical absorbance profilethan we observe. Upon the basis of the total Au/Ag particlesize of 27 nm, the silver shell thickness was about 7 nm, muchlarger than the total particle size of our largest Au/Ag samples.The larger shell thickness may provide more effective electro-magnetic screening of the Au core and likely a spatially sharperAu/Ag interface relative to the total particle size. This does notrule out some interdiffusion of atoms at the Au/Ag interfacefor both small and large nanoparticles as was suggested in thework of Shibata and co-workers.12 They found that atomicinterdiffusion was greatest for the smallest Au-core particlesstudied, 4.6 nm, and thus of direct relevance to the even smallerAu-core particles of the present study. Shibata et al. reportedthat for particles such as the 12 nm Au-core particles of Lu etal. that the Au/Ag interface is sharp to within a monolayer. Theamount of Au/Ag atomic interdiffusion was found to dependstrongly on the presence of defects such as vacancies at theinterface in their model. If such vacancies are annealed naturallyduring the cluster deposition/growth process, then a sharperinterface would be predicted, even for smaller particles.

The optical absorbance features of AgAu alloy nanoparticlesare shown in Table 2 and in Figure 7 for three compositions ofsimilar sizes between 4.3 and 5 nm. The absorbance spectrumof a reference Au, d ) 4 nm particle is also plotted. Opticalproperties for smaller 3 nm AgAu nanoparticles are alsopresented in Table 2. The spectra in Figure 7 have been offsetvertically for easier comparison. As the ratio of Ag/Au increasesfrom 2:1 to 1:2 the peak red-shifts from 487 to 511 nm. Thelatter is within experimental error of the peak of the Au, d )4.0 nm sample shown in Table 2. The half-width to the red ofthe peak broadens from 59 to 86 nm as the Au content increases.The 3 nm AgAu nanoparticles also red-shift and broaden withAu content, but for each composition have more broadening,as is common for smaller particles of both Au and Ag.7

Chromatography and TEM indicate that all these AgAunanoalloy samples have low size dispersion of between 5 and

10% of the average size and a spherical shape. Figure 8 showson highly ordered region of the AgAu, 1/2, d ) 4.3 nm sample.Facets on individual nanoparticles can be seen in this image.Comparing the nanoparticle electron diffraction contrast ofFigure 8 to that of Figure 3 of the Ag/Au, 1/2, d ) 4.8 nmsample, we can see that the center region of many of thenanoparticles in Figure 3 appear to be lighter or lower contrast,as might be expected if the lower Z metal Ag is concentratedin the core. However, because all nanoparticles on the grid havea wide range of orientation relative to the incident electron beam,the diffraction contrast varies considerably from particle toparticle in both TEMs. When combined with the data of Table2, showing the latter sample to have more line broadening, thecore/shell structure is suggested, but it not possible to rule outthe minority presence of Au atoms in the core nor Ag in theshell.

Figure 9a, left panel, shows a TEM of the Au, d ) 4 nmsample, and the right panel (b) is a TEM of the Au/Ag, 1/1, d) 3.7 nm sample, whose optical properties are given in Table2. There is no evidence from panel b that Ag atoms areconcentrated in the shell, and the differences in diffractioncontrast between nanoparticles in each TEM due to particleorientation effects are as great as between panels. So, one wouldnot be able to identify the nanostructure from these TEMs,although the optical absorbance is significantly different, asnoted previously. For example, the Au/Ag, 1/1, d ) 3.7 nmsample has less absorbance on the short wavelength side of thepeak due the presence of the Ag in the shell.

The identical gaps between particles in Figures 3, 8, and 9are due to the presence of dodecanethiol on the cluster surface.Without a surfactant on the cluster surface, cluster aggregationwill occur. The effect of such aggregation on the opticalabsorbance is a significant red shift of the absorbance peak tothe 600-700 nm range and broadening of the absorbance inthe case of Au nanoclusters.26 There is no evidence from eitherTEM or SEC of any of the samples of Table 2 that aggregationplays a role in the optical absorbance properties.

Figure 7. Absorbance spectra obtained at the elution peak for threeAgAu nanoalloy samples and one reference Au sample. Labels on thespectra are as follows: AgAu, 2/1, d ) 5.0, open circles, red curve;AgAu, 1/1, d ) 4.8, open squares, blue curve; AgAu, 1/2, d ) 4.3,open triangles, green curve; and Au, d ) 4.0, solid circles, black curve.

Figure 8. Bright-field TEM of a nanoalloy sample, AgAu, 1/2, d )4.3 nm. The nanoparticles are stabilized with dodecanethiol on thecluster surface and form ordered hexagonal arrays on the holey carbongrid.

2654 J. Phys. Chem. B, Vol. 113, No. 9, 2009 Wilcoxon

Several groups have investigated AgAu nanoalloys with amore-or-less random distribution of Ag and Au atoms. Larger,charge-stabilized clusters in water, as well as smaller, stericallystabilized clusters in organic solution were investigated. Acommon finding is that only a single absorbance peak ismeasured and that its position shifts nearly linearly withcomposition from that of a pure Ag cluster absorbance at ∼420nm to pure Au absorbance at ∼520 nm. Mallin and Murphy,for example, reported that AgAu clusters with sizes between 5and 6 nm had absorbance peaks in water at 430, 465, and 505nm for compositions of Ag/Au ) 3/1, 1/1, and 1/3, respec-tively.10 Their spectra also show very significant broadening,(e.g., >100 nm for AgAu, 3/1 composition), exceeding thatobserved in our AgAu, 2/1 sample (59 nm). It is possible thattheir growth process in water produces more defects at theinterface, leading to more damping and electron scatteringenergy loss and thus additional line broadening. Link and co-workers, synthesized larger, ∼10 nm AgAu nanoparticles inwater and also observed only a single plasmon absorbance peakwhose position depended linearly on composition.4 Perhaps dueto the larger overall cluster size, their linewidths were consider-ably narrower, about 50 nm for their AgAu, 1/1 samplecompared to the Mallin studies. This reduced broadeningindicates reduced energy loss at the nanoparticle/water interface.The actual peak positions for a given composition were nearlyidentical, showing a negligible size dependence for AgAunanoparticles in water. We observe only a slight blue shift forthe AgAu nanostructure with decreasing size, similar inmagnitude to that reported previously for pure Au nanoclusters.7

Charge stabilized AgAu clusters in water may have opticalspectra that differ from clusters of similar size and compositionthat are neutral and sterically stabilized in organic solvents suchas toluene or hexane. A study by Kariuki and co-workers of2-3 nm AgAu nanoclusters synthesized by a two phasereduction of AuCl4 and AgBr2 and stabilized by dodecanethiolcan give insight into this question and allow comparison to ourdodecanethiol-stabilized 3 nm and 4-5 nm AgAu clusters.14

As in the studies of larger AgAu clusters in water, a red-shift,

loss of peak symmetry, and broadening was observed withincreasing Au content. Instead of a linear dependence of peakposition on Au content over the full range of Au, the peakabsorbance first red-shifted for compositions less than Ag/Au) 1:2 (∼66% Au) and then saturated at 520 nm for higher Aucontent. This is similar to our observations obtained during SEC,and the observed peak position of our AgAu, 1/1, d ) 3.2 nmof 491 nm is identical to that reported by that group. The peakof 506 nm for our AgAu, 1/2, d ) 3.1 nm is also close to thevalue of 520 nm found in their experiments. A pure 3 nm Aucluster has an absorbance maximum at 500 nm.7

IV. Conclusions

We used SEC to size separate and analyze nanoparticles withtwo types of nanostructure, core/shell and alloy, as a functionof composition and size. We find a very weak dependence ofplasmon energy and damping in nanoalloys compared to core/shell structures of equal size. The nanoalloys show a blue shiftwith decreasing size, comparable to that observed for pure Auclusters. We also observed that increasing the Au content ofAg/Au core/shell nanoparticles leads to a red shift and increasedbroadening of the plasmon absorbance, whereas an increase inAg content in Au/Ag nanoparticles results in a blue shift andreduced peak width. In both cases, the larger amounts of Au inthe nanoparticle increases the absorbance at wavelengths to theblue of the peak possibly due to the importance of interbandtransitions in Au compared to Ag. For a constant compositionand size, each type of nanostructure can be distinguished andidentified solely by its optical absorbance features.

Acknowledgment. This work was partially performed atSandia National Laboratories and supported by the Division ofMaterials Sciences, Office of Basic Energy Research of the U.S.Department of Energy under Contract No. DE-AC04-94AL8500.

References and Notes

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Figure 9. Bright-field TEM of the (a) Au, d ) 4 nm and (b) Au/Ag, 1/1, d ) 3.7 nm core/shell sample, both stabilized with dodecanethiol andimaged with the same magnification and resolution.

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